Photo by Aleksandar Milosevic
Photo by Aleksandar Milosevic

Binoculars, or field-glasses, are a pair of telescopes mounted together to be hand-held, for use by travellers, soldiers, sailors, birdwatchers, and sports spectators.  A binocular is an optical device consisting of two parallel telescopic systems that enable the user to see an enlarged image of a distant object with both eyes.

In addition to giving an enlarged view of a distant object, the binocular enhances the stereoscopic perception of depth beyond that of normal vision. This enhancement is aided both by the separation of the axes of the objective lenses and by the magnifying power. The term "binocular" can be applied to any device using the parallel telescope prin ciple, but it is commonly restricted to prism binoculars- instruments that use prisms to erect the image and to shorten the distance between the objective (front) lens and the ocular (rear) lens. Each half of a binocular contains two prisms and at least two lenses.

Binoculars are specified by first giving the magnifying power in diameters and then giving the diameter of the objective lens. Thus, the expression "7 x 50 binocular" tells the user that the instrument has a magnifying power of 7 diameters, a clear objective lens aperture of 50 millimeters, and prisms in the optical path.

The intended use of a binocular determines the appropriate size and power. A large-diameter objective lens provides large light-gathering power, which is useful at low levels of illumination, such as at twilight. Thus a 7 x 50 instrument is eminently suitable for such conditions. For viewing sports events or for bird watching, a 6 x 30 or a 7 x 35 binocular has adequate power and is likely to be relatively lightweight. A fixed, stable mount should be used to support high-power (8 or more) binoculars to prevent shake of the image.

The Galilean binocular or opera glass, similar in principle to Galileo's telescope, is the simplest form; it is used in magnifications of 2 to 3 times, mainly for viewing in the theatre. The prism binocular can be used at magnifications up to about 12, beyond which it cannot be held steady enough in the hands. The prisms referred to are for producing an image which is right side up; the first system for doing this was invented by Porro, an Italian engineer, but several other prism systems are now also used.

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